A phase-cut dimmer is a conventional electrical device designed as a simple, efficient, and inexpensive apparatus to adjust a light output of an incandescent light source (e.g., to allow for dimming). Such a dimmer operates by limiting the power delivered to the light source by only conducting current for a certain portion of each half-cycle of an AC line voltage. The dimmer may be adjusted (e.g., by turning a knob or changing the position of a slider) to vary the portion of the AC line voltage half-cycle during which the dimmer conducts current, thereby varying the power provided to the light source to increase or decrease the light output of the light source.
There are two different types of conventional phase-cut dimmers. A “leading-edge dimmer” delays the conduction period of the dimmer until after a zero crossing of the AC line voltage, thereby cutting out the initial portion of each half-cycle and conducting during the later portion of each half-cycle. In contrast, a “trailing-edge dimmer” operates in the opposite manner, i.e., conducting during the initial portion of each AC half-cycle after a zero crossing and cutting out during the later portion of each half-cycle. Leading-edge dimmers are generally used for inductive loads (e.g., magnetic low voltage transformers) whereas trailing-edge dimmers are generally used for capacitive loads (e.g., electronic low voltage transformers, LED drivers). Both types of dimmers may be used for resistive loads (e.g., incandescent lights).
Leading-edge dimmers are generally less expensive and have a more simple design than trailing-edge dimmers, and are conventionally used to adjust the light output of incandescent and halogen bulbs. These types of dimmers employ a triac switch to control the power provided to a light source, and hence are often referred to as “triac-based dimmers” or simply “triac dimmers.” Triac-based dimmers are the most common type of dimmers conventionally used for dimming light sources.
FIG. 1 illustrates a conventional triac-based dimmer 100, showing an input AC line voltage VLINE 105 and a dimmer output VDIM 110. The dimmer 100 includes a triac, a diac coupled to a gate of the triac, a resistor R1, a capacitor C, and an adjustable resistor R2 (which facilitates an adjustment of the dimmer to vary the dimmer output 110, via a knob or slider for example). In FIG. 1, when the dimmer 100 is connected to the AC line voltage 105, the voltage VDIM charges the capacitor C to a voltage VRC by conducting current through the adjustable resistor R2 and the resistor R1. When VRC reaches a breakover voltage of the diac, a voltage is applied to the gate of the triac and the triac begins conducting the current ITRIAC 115. The resistance of the adjustable resistor R2 determines the time required to charge the capacitor C to the diac breakover voltage (e.g., a smaller resistance for R2 results in faster charging times for capacitor C, and a larger resistance for R2 results in slower charging times for capacitor C). Accordingly, the resistance of R2 determines when the triac begins conducting the current ITRIAC during each half-cycle of the AC line voltage, and thus adjusting the resistance of R2 varies the power provided by the dimmer output 110.
FIG. 2 illustrates the input line voltage 105, the dimmer output 110, the triac current ITRIAC 115 and a triac holding current IHOLD 120 of the triac-based dimmer 100 of FIG. 1. In FIG. 2, the resistor R2 is adjusted such that the triac begins to conduct the triac current ITRIAC 115 and provide the dimmer output VDIM 110 after a zero crossing and during the first half of a half-cycle of the AC line voltage 105. As also shown in FIG. 2, the triac stops conducting the triac current ITRIAC 115, and the dimmer output VDIM 110 goes to zero, when the triac current ITRIAC 115 is less than the triac holding current IHOLD 120 (the triac holding current is thus defined as the minimum current at which the triac conducts current). Conventional triac-based dimmers from a variety of manufacturers may have triac holding currents that vary significantly from manufacturer to manufacturer and model to model; for example, for an AC line voltage 105 having a nominal value of about 120 VRMS (plus or minus 10%), the triac holding current IHOLD 120 for a given triac-based dimmer 100 may be in a range of from about 5 milliamperes (mA) to 20 mA.
FIG. 3 illustrates example waveforms of a rectified dimmer output voltage 125 (in which the dimmer output 110 is applied to a rectifier to invert alternate half-cycles and thereby provide the rectified dimmer output voltage 125). As shown in FIG. 3, the rectified dimmer output voltage 125 has different phase angles as the triac-based dimmer 100 of FIG. 1 is adjusted (i.e., as the resistance of the adjustable resistor R2 is adjusted). The point in each half-cycle at which the triac of the dimmer 100 begins to conduct triac current ITRIAC 115 (and thus provided the rectified dimmer output voltage 125) is conventionally referred to as the “firing angle” or the “conduction phase angle” (or simply “phase angle”) of the dimmer. In FIG. 3, multiple waveforms are illustrated for comparison to show different phase angles for different dimmer adjustments; on the left of FIG. 3, there is a full rectified waveform of the AC line voltage (corresponding to a theoretical phase angle of 180 degrees). Immediately to the right of this waveform, a dimmer adjustment is shown that results in a phase angle of 135 degrees (in which the first 45 degrees of each half-cycle is “cut off” when there is no triac current ITRIAC 115). FIG. 3 also shows additional waveform examples corresponding to phase angles of 100 degrees and 30 degrees, respectively (which provide relatively lower power from the dimmer).
As with the triac holding current, conventional triac-based dimmers from a variety of manufacturers may have maximum and minimum phase angles that vary significantly from manufacturer to manufacturer and model to model; consequently, the range of conduction periods and power delivered to a load may vary from dimmer to dimmer. For example, minimum phase angles for conventional triac-based dimmer may be in a range from 17 degrees to 72 degrees, and maximum phase angles may be in a range of from 104 degrees to 179 degrees.
It is conventionally difficult to effectively dim an LED light source to relatively low light output levels with triac-based dimmers that were originally intended for incandescent lights. Triac-based dimmers are not readily compatible with LEDs since LEDs do not appear as a resistive load. Accordingly, a problem for LED light sources employed in retrofit light fixtures intended to replace older incandescent fixtures is that often there are triac-based dimmers already installed in the environment for dimming of the legacy light fixture(s)—and these triac-based dimmers may not function appropriately with the replacement/retrofit LED light sources.
An LED driver generally is required in (or in connection with) a light fixture including an LED light source to provide power to the LEDs from a conventional source of wall power (e.g., an AC line voltage, 120 VRMS/60 Hz). There are conventional LED driver solutions that allow for triac-based dimmers to be used with LED light sources. These conventional LED drivers generally provide adjustment of the output power to an LED light source using pulse width modulation of a power converter (e.g., a buck converter or a flyback converter). Examples of conventional LED drivers that allow for triac-based dimming of LED light sources employ specialized integrated circuits provided by various manufacturers, examples of which include the National Semiconductor LM3450, the Texas Instruments TPS92210, the Linear Technology LT3799 and the Fairchild/ON Semiconductor FL7734.
FIG. 4 is a block diagram of a conventional single-stage primary-side-regulation pulse-width-modulation-controlled LED driver for use with a triac-based dimmer, and FIG. 5 is a circuit diagram for the conventional LED driver shown in FIG. 4 based on the Fairfield/ON Semiconductor FL7734 integrated circuit. Details of the LED driver shown in FIGS. 4 and 5 may be found in the ON Semiconductor technical documentation entitled “LED Driver with Phase-Cut Dimmable Function, 8.6W,” Publication Order No. TND6251/D, dated January 2018, and the Fairchild/ON Semiconductor technical documentation entitled “FL7734 Single-Stage Primary-Side-Regulation PWM Controller for PFC and Phase Cut Dimmable LED Driving,” Publication FL7734, Rev 1.0, dated November 2014, both of which publications are hereby incorporated by reference herein in their entirety.
As shown in the block diagram of FIG. 4, the conventional LED driver employing the FL7734 integrated circuit is employed to control (increase or decrease) a light output 2052 of an LED light source 2050 (e.g., including one or more LEDs) via adjustment of the triac-based dimmer 100, which provides the triac current ITRIAC 115 and the dimmer output 110. An EMI filter and surge protection circuit 200 is employed to attenuate common mode and differential mode noise that may be generated within the driver, as well as to provide transient voltage suppression by attenuating line surges and electrical fast transients (e.g., in the AC line voltage). Rectifer 300 provides the rectified dimmer output voltage 125 based on the dimmer output 110.
In FIG. 4, the LED driver includes a power converter 600 (e.g., a flyback converter) that includes a transformer having a primary winding 612, a secondary winding 614, and an auxiliary winding 610 (e.g., to provide operating power for the FL7734 integrated circuit). The power converter also includes a snubber circuit 604 to suppress voltage spikes caused by the primary winding inductance during switching operation of the power converter (discussed below). The primary winding 612 is coupled to the rectified dimmer output voltage 125 (e.g., through a post EMI filter 500), and the secondary winding 614 provides an output power (e.g., low ripple DC average voltage and current) to the LED light source 2050 (via the operation of diode 606 and capacitor 608). Based on the configuration of the flyback power converter, an average output current 2054 (also referred to as “secondary-side current”) generated in the secondary winding 614 of the transformer (and conducted by the LED light source 2050 to generate light output 2052) is related to an average primary current 150 (conducted through the primary winding 612 of the transformer) though a turns ratio of the primary winding and the secondary winding of the transformer.
The instantaneous current conducted through the primary winding 612 of the transformer of the flyback converter 600 in FIG. 4 is governed by a controllable switch 602 (e.g., a MOSFET) that receives a pulse-width-modulated (PWM) control signal (Gate) from a PWM controller 900 (which includes the FL7734 integrated circuit, as shown in FIG. 5). In general, the duty cycle of the PWM control signal provided to the switch 602 by the PWM controller 900 determines the magnitude of the average current 150 conducted on the primary side, which as noted above determines the average output current 2054 to the LED light source 2050 (via the turns ratio of the primary and secondary windings of the transformer). The duty cycle of the control signal provided by the PWM controller 900 depends on multiple factors, such as: 1) the dimmer output 110 (as sensed by the dimmer output voltage sensing block 700 to provide the sampled voltage VIN to the PWM Controller 900); 2) the current through the primary winding (as sensed by the primary current sensing block 1010 to provide the signal CS to the PWM Controller 900); and 3) the secondary-side output voltage across the LED light source (as sensed by the output voltage sensing block 1020, which divides a voltage across the auxiliary winding 610, representative of the voltage across the secondary winding 614, and provides the signal VS to the PWM Controller 900). By way of example, a maximum value for the sampled dimmer voltage VIN is approximately 24 V, a maximum value for a peak voltage at CS is approximately 1.2 V, and a maximum value for the sensed voltage VS is approximately 6 V. As noted above, the auxiliary winding 610 of the transformer also provides an operating voltage VDD for the PWM Controller 900 (a nominal value for VDD is in the range of 16-24V).
The conventional LED driver circuit shown in FIGS. 4 and 5 also includes a start-up active bleeder block 800 to facilitate rapid power-up operation of the PWM controller 900 during a power-on start-up sequence. In particular, the active bleeder block 800 couples the rectified dimmer output voltage 125 to the PWM controller operating voltage VDD (by quickly raising the Bias voltage from the PWM controller as soon as there is some dimmer output 100) to conduct a current through the start-up active bleeder for a short time (e.g., on the order of 4 to 5 half-cycles of the dimmer output). After this brief start-up sequence, the start-up active bleeder block is deactivated.
The circuit shown in FIGS. 4 and 5 also include a passive bleeder block 400 to provide a current path for a passive bleeder current 155 across the rectified dimmer output voltage 125 of the rectifier 300. As shown in FIG. 5, the passive bleeder block 400 across the output of the rectifier 300 includes a resistor and capacitor in series across the rectified dimmer output voltage 125; as noted in the ON Semiconductor technical documentation entitled “LED Driver with Phase-Cut Dimmable Function, 8.6W,” Publication Order No. TND6251/D, dated January 2018, a nominal value for the resistor in the passive bleeder block 400 is 500 ohms and a nominal value for the capacitor in the passive bleeder block is 0.15 microfarads (150 nanofarads). The RC circuit of the passive bleeder block 400 provides a complex (frequency-dependent and non resistive-like) impedance across the output of the rectifier, including a resistive component and a capacitive component (reactance).
The conventional role of the passive bleeder block 400 in FIGS. 4 and 5 is to provide the passive bleeder current 155 as at least a portion of the triac current ITRIAC 115 in an effort to maintain the triac current ITRIAC above the triac holding current IHOLD 120. The passive bleeder current 155 is a more significant component of the overall triac current ITRIAC 115, particularly at lower light output levels (also referred to as “deeper dimming”), when the output current 2054 to the LED light source is relatively lower (and, accordingly, the average primary current 150 is relatively lower). As long as there is a dimmer output 110, however, the passive bleeder circuit 400 conducts some passive bleeder current 155, which performs essentially no salient function at relatively higher light output levels (when the average primary current 150 is significantly above the triac holding current); thus, under these circumstances, the passive bleeder block 400 continues to use power and decreases the efficiency of the driver.